Moscovici A. High Speed A/D Converters. Understanding Data Converters Through SPICE2

Kluwer, 2000. — 239 p.The Analog to Digital Converters represent one half of the link between the world we live in - analog - and the digital world of computers, which can handle the computations required in digital signal processing. These devices are mathematically very complex due to their nonlinear behavior and thus fairly difficult to analyze without the use of simulation tools. Fortunately the availability of home computers, mathematical and circuit simulation programs can make this task easier. This book attempts to present the subject from the practicing engineer rather then the academic point of view so a practical approach is provided to the topic.
This book is intended to shed some light on the intricacies of various topologies of A/D converters. It is also intended as a learning tool by providing building blocks that can be stacked on top of each other to build higher order systems. The book provides a guide to understanding the various topologies used in A/D converters by suggesting simple models for the blocks used in an A/D converter. The converters discussed through the book constitute a class of devices called undersampled or Nyquist converters. The subject of oversampled A/D converters which are capable of achieving high degree of resolution with small amount of hardware based on a sigma-delta (Σ-Δ) modulator is not discussed however. These Σ-Δ converters sample the analog input repeatedly and quantize the signal one bit at a time. Since the quantization process of the Σ-Δ A/Ds is limited by the nature of their algorithm these converter are mostly used in low speeds applications.
Another reason for this book stems from the user’ need to understand the key limitations in converter accuracy. The majority of converters used today are integrated circuits (I.C.) where the user does not have access to internal nodes. Some of the high accuracy converters require laser trim in order to attain higher matching of the internal components. The trim process is a mechanism used to alter a resistor value so it can match another resistor performing a similar function elsewhere in the IC. The trim process damages the resistor physical structure and in time, the matching obtained initially is reduced. The aging process is therefore the reason that A/D performance degrades over time.
Most manufacturers do not supply their customers with extensive models of their A/D converters. Thus the system designer is left without the capability to synthesize his system while performing a “what if” scenario. This invariably causes the system to be over-specified and contributes to the cost escalation of the product. Usually the A/D converters are treated as “black boxes”, but a better appreciation of what makes the converters “tick”, what makes them operate properly and the effects of parasitic loading on the various nodes can give the reader a better understanding of how to use them. To this end the book is making extensive use of the ubiquitous SPICE program which was developed in the early 1970 as an extension of CANCER and SPICE1 (Simulation Program with Integrated Circuit Emphasis) at Berkeley (ref. 69). In suggesting the use of SPICE the goal is to allow the reader to experiment and learn through a process of trial and error thus minimizing the frustration of system loading in the troubleshooting process. The use of SPICE is a powerful complement to the breadboarding phase in solving problems in almost any circuit. SPICE can help with the test of circuit operation theory, behavioral modeling and component functionality under a variety of temperatures, component tolerances and power supply conditions. The key to the proper use of the SPICE simulator as a tool is to enhance the speed an accuracy of the simulation.
In using SPICE, the analysis of large circuits can prove to be a difficult task even for the computers available today. Sometimes SPICE aborts the simulation and proclaims the infamous nonconvergence error message (ref.69). The key ingredient of the SPICE analysis is to be able to observe relevant details of the design in a sensible amount of time. This text will suggest some of the techniques necessary to reduce the analysis time in the face of increased complexity. The main contributor to the simulation length is the number of circuit nodes. For this reason, it is very important to develop a simple behavioral model that represents the circuit response as a function of its linear as well as nonlinear behavior. The general scope of the macro-model we develop is to have the minimum number of nodes in the circuit, such that the simulation time is minimized.
Several macro-models will be presented along the way illustrating various A/D topologies with the associated simulations making the reader familiar with each topology pitfalls under different transient conditions. The reader will become accustomed with the specific behavior of each of the topologies employed and will be able to recognize the “signature” of the various topologies. The book supplies not only the models for the A/D converters for the SPICE program but describes the physical reasons for the converter’s performance.A/D Terminology
The Comparator
Flash A/D
Track and Hold Amplifier
SAR A/D
Folding A/D Converters
Pipelined A/D Converter
Serial Pipeline A/D with 1.5-Bit/Stage